1. Field of the Invention
The present invention relates to a display device having an optical modulation element for modulating source light in a two-dimensional area and, more particularly, to a display device used for displaying moving images in tones.
2. Description of the Related Art
In recent years, a liquid crystal display panel having a high brightness level and a wide viewing angle has been developed. This liquid crystal display panel is an optical modulation element including a pair of transparent substrates in which a matrix of pixel electrodes and a single common electrode are respectively arranged, and a polymer dispersed liquid crystal cell having a polymer resin containing a liquid crystal material and held between the substrates or a fine particle dispersed liquid crystal cell having a liquid crystal material containing fine particles and held between the substrates. In this liquid crystal display panel, the light scattering property of the liquid crystal cell is controlled by a drive voltage applied between the common electrode and each pixel electrode. For example, when the liquid crystal cell is set in an opaque light scattering state at a drive voltage of 0 V, the transmittance of the liquid crystal cell increases with an increase in the drive voltage, and the liquid crystal cell is finally set in a transparent light-transmission state.
For example, a polymer dispersed liquid crystal cell (PDLC) has the following drawbacks. FIG. 1 shows the modulation characteristics, i.e., the transmittance-to-voltage characteristics of the PDLC. FIG. 2 shows the response characteristics of the PDLC, i.e., the relationship between the response time to a change in the drive voltage. Referring to FIG. 2, the voltage axis indicates an initial level of a change in the drive voltage, reference symbols V1 to V8 respectively indicate final levels of the change in the drive voltage. In this case, in order to easily understand the modulation characteristics of the PDLC shown in FIG. 1, FIG. 3 exaggeratedly shows the modulation characteristics shown in FIG. 1. As is apparent from FIG. 3, the modulation characteristics of the PDLC have hysteresis in which the transmittance depends on the direction of the change in the drive voltage. More specifically, the transmittance changes along a characteristic curve CV1 when the drive voltage increases, and the transmittance changes along a curve CV2 when the drive voltage decreases. Therefore, two transmittance levels exist for a level P of the drive voltage.
When the liquid crystal display panel displays moving images in tones, images for successive field are overlapped due to the hysteresis. In addition, the response of the PDLC is considerably delayed when the drive voltage is changed for designating halftones between white and black corresponding to the transmittance of 0% and 100%. This is a factor for forming an afterimage when the liquid crystal display panel displays moving images in tones. In addition, a threshold value Vth of the PDLC easily changes depending on a change in temperature. Due to the above problems, in the polymer dispersed liquid crystal display panel it is hard to obtain good image quality in a case where the polymer dispersed liquid crystal display panel is used as a TV display.
A TN liquid crystal display panel has been popular prior to the above polymer dispersed liquid crystal display panel. A typical TN liquid crystal display panel comprises a pair of transparent substrates in which a matrix of pixel electrodes and a single common electrode are respectively arranged, a liquid crystal cell held between these substrates, and a pair of polarizing plates affixed to the outer surfaces of the substrates with a phase difference of 90 degrees. An alignment process is performed on the surfaces of these substrates which are set to be in contact with the liquid crystal cell, in order to obtain a TN-alignment of liquid crystal molecules on the axis extending in the thickness direction of the liquid crystal cell. When source light is linearly polarized by one polarizing plate and then incident on the liquid crystal cell, the polarized light is rotated through 90 degrees and guided to the other polarizing plate. When a drive voltage is applied between the common and one of the pixel electrodes, the liquid crystal molecules are tilted up in an area between these electrodes to obtain birefringent polarized light so as to prevent the polarized light from passing through the other polarizing plate.
However, when this liquid crystal display panel is used as a TV display, the response characteristics of the liquid crystal cell pose a problem. Conventionally, various attempts have been made to improve the response characteristics.
The response characteristics of a liquid crystal cell depend on a time tr required for tilting up liquid crystal molecules in an electric field generated by a drive voltage applied between a pixel electrode and a common electrode and a time td required for returning the alignment of the liquid crystal molecules to the original state by a force acting between molecules when the electric field is removed. The times tr and td are expressed by the following equations: EQU tr=.eta.d.sup.2 /(.DELTA..epsilon.V-K.pi..sup.2) (1) EQU td=.eta.d.sup.2 /K.pi..sup.2 ( 2)
In this case, K is a constant expressed by K=K.sub.1 +(K.sub.3 -2K.sub.2)/4, where the elastic constants of the divergence, torsion, and bending of the liquid crystal are represented by K.sub.1, K.sub.2, and K.sub.3, respectively; .DELTA..epsilon. is a difference (.epsilon..sub.s -.epsilon..sub.p) between a parallel dielectric constant .epsilon..sub.s of the liquid crystal molecules and a perpendicular dielectric constant .epsilon..sub.p of the liquid crystal molecules; .eta. is the torsional viscosity of the liquid crystal molecules; d is the thickness of the liquid crystal cell (the gap between the substrates); and V is a drive voltage applied between the pixel electrode and the common electrode to tilt up the liquid crystal molecules. As is apparent from equations (1) and (2), the response time of the liquid crystal cell can be shortened by decreasing the constant .eta., decreasing the thickness d, or increasing the constant K. However, the constants .eta. and K are inherent to a material, and the thickness d cannot be minimized because the minimum transmittance is determined by the relationship between the thickness d and a difference .DELTA.n indicating the anisotropy of a refractive index. For this reason, an effort for changing the constants .eta. and K, the difference .DELTA.n, and the like has been continued by blending various liquid crystal materials with each other. The tilt-up time tr can be shortened by changing the difference .DELTA..epsilon. or the voltage V. The tilt-down time td can be shortened by a known technique of applying a high-frequency voltage between the electrodes when the drive voltage V is removed, in order to set the anisotropy of a dielectric constant to be negative.
The countermeasures described above are effective only when the liquid crystal cell is used to display images in two tones of white and black, which are set in the states where light is transmitted and blocked, respectively. When the liquid crystal is used to display images in tones including halftones between white and black, a more complex situation must be considered.
FIG. 3A shows one liquid crystal molecule 3 which is present between electrodes 1 and 2 in the TN liquid crystal display panel. Referring to FIG. 3A, reference symbol .theta. denotes an angle between the liquid crystal molecule 3 and an x-axis on an X-Y plane parallel to the electrodes 1 and 2, and reference symbol o denotes an angle between the liquid crystal molecule 3 and a z-axis perpendicular to the X-Y plane. When a drive voltage is applied between the electrodes 1 and 2, an electric field in the z-axis direction is applied to the liquid crystal molecule 3. At this time, hydrodynamic equations of the liquid crystal molecule 3 are as follows: ##EQU1##
Although these equations are nonlinear partial differential equations which cannot be analytically solved, these equations can be solved by numerical calculations. In addition, a drive voltage V applied between the electrodes 1 and 2 is expressed by the following equation: ##EQU2## where a=(.epsilon..sub.s -.epsilon..sub.p)/.epsilon..sub.p, and D.sub.z is an electric flux density.
As is apparent from equations (3) to (5), the inclination of the liquid crystal molecule depends on the drive voltage which changes with time. The transit angles .theta.(z, t) and o(z, t) of the liquid crystal molecule can be obtained by simultaneously solving equations (3) to (5). The response characteristics of the liquid crystal molecule can be finally derived such that the angles .theta.(z, t) and o(z, t) obtained as described above are substituted in a Barrman 4.times.4 matrix and this matrix is solved.
On the other hand, FIG. 3B shows modulation characteristics representing the relationship between the drive voltage and the transmittance of the liquid crystal cell. Referring to FIG. 3B, assuming that the liquid crystal cell is normally white, the drive voltage requires an amplitude of about 5 V to obtain a contrast ratio of 100/1. When only the halftone range between black and white is considered, the drive voltage is set to an amplitude of 1.5 to 2 V. This means that an image display is delayed in the halftone range.
This delay poses a problem when the liquid crystal display device is used as a full-color TV display. The full-color TV display requires a liquid crystal cell whose response time is less than 10 msec in the halftone range. However, the response time of existing liquid crystal cells exceeds 20 msec even if images are displayed in two tones of white and black. This result in that a considerable afterimage phenomenon occurs when the liquid crystal display cell is used to display moving images.